Handpieces for dental or surgical applications are used for a multitude of clinical procedures. Carrying out these procedures is done with a large selection of burs. Changing the burs during a procedure or in between procedures is done through a mechanism in the head of the handpiece which is activated by a push button. To release/introduce a bur, the push button is depressed (pressed down) to activate a release mechanism of the chuck. During a procedure, in particular a dental procedure, the handpiece is introduced in the oral cavity, for example to remove caries. To access a tooth from the vestibular side, the soft tissue (cheek) must be pushed away, in order to create the space necessary for the handpiece and the bur. If this is done using the handpiece itself, there is a risk that the push button is depressed during the procedure while the bur is rotating, thus creating friction between the rotating part of the chuck and the non-rotating push button. With a rotation speed in the range between 40,000 and 400,000 rpm, heat builds up very quickly with the risk of burns to the soft tissue.
The published patent application FR2679804A1 of the applicant related to a handpiece describes an early approach to solve the problem of overheating of the push button by means of stainless steel bearing ball mounted on the rotating part of the chuck. The push button enters in contact with the ball when being depressed. The small contact surface (theoretically a single point) between bearing ball and push button limits the build-up of heat in the push button even if the chuck is rotating. In order to limit wear-out of the push button, a small washer of hard-metal has been soldered to the inner side of the push button, delaying heat build-up. Nevertheless, the friction will end up generating heat in the hard metal washer which transmits the heat directly to the push button and potentially to the patient. Finally, the stainless steel ball will wear out with time, increasing the contact surface with the push button.
Several alternative approaches have been developed to overcome the problem such as:                integrating a ball bearing in the Push-Button and using the inner and outer ring of the ball bearing to insulate the push-button from heat-transmission. However, in addition to being an expensive solution, the ball bearing is used in a way that leads to fast wear-out of the ball bearing;        Providing the chuck mechanism with a rounded end in hard metal. However, due to the large geometry, the heat protection of this design is very limited.        